In the recent years we have come to realize that several threats

In the recent years we have come to realize that several threats to the environment are fundamental. One is acid rain,which is created by the millions of tons of sulphur dioxide and nitrogen oxides spewed out of North American smokestacks and automobile exhaust pipes.The oxides mix with water vapor in the air to form weak sulphuric and nitric acid,which later falls as acid rain.The result is increased acidity in lakes,which has curtailed the ability of many fish to reproduce,and in the soil,which has slowed the growth of trees and increased their vulnerability to disease.
【正确答案】：

近年未我们逐渐认识到，有几种物质对环境构成的威胁是根本性的，其中之一便是酸雨。酸雨现象是由大量二氧化硫和氧化氢造成的。在北美洲，大烟自和汽车尾气管每天都排放出大量的二氧化硫和氧化氮。这些氧化物在空气中和水蒸气结合，转化为弱硫酸和弱氮酸，降落到地面便形成酸雨。酸雨现象不但导致湖泊酸性提高，使鱼类繁殖能力下降；而且随着土壤里酸性物质的激增，树木的生长速度开始放慢，对疾病的抵抗力明显下降。

【解析】原文共四句话，基本采取直译法，但句子结构做了调整。第一句是主从复合句，that 引导宾语从句，原句中fundamental修饰的是threats，在译文中增译“物质”，使译文更通顺。第二句话是主从复合句，one is acid rain编进第一句话，符合汉语的表达习惯；which引导的非限制性定语从句修饰acid rain，单独译作一句话spewed out of.…是过去分词作后置定语，译文将其转化为谓语“排放出”。第三句话中to form weak sulphuric and nitric acid在句中作结果状语，译文译作并列谓语；which引导非限制性定语从句修饰acid，译文也单独译作一个分句。最后一句话是主从复合句，主句是主系表结构，但译文转化为主谓宾结构；两个Which引导的从句分别修饰acidity in the lakes 和acidity in the soil。